A potentially game breaking bug has been reportedly found in Pokemon Sword and Shield preventing online play.
The bug involves the Surprise Trade feature within Pokemon Sword and Shield whereby if you're sent a "maliciously crafted" Pokemon access to online play will be blocked. This applies to all online features and you won't be able to regain access to them until the bug is patched.
Avoid Surprise Trades
The Pokemon causing the issue were described as "maliciously crafted" by renowned dataminer Kaphotics. Presumably they haven't elaborated on this any further in an attempt to ensure the problem doesn't become more widespread.
POC for the Surprise trade Crash! pic.twitter.com/o6CtylL0mo
— thecommondude (@architdate) February 12, 2020
Until further details are revealed or a patch is released, we'd strongly recommend to avoid using the Surprise Trade feature. There have been multiple reports of the issue but just how common it is, is currently unknown.
Your save data is safe
The bug is obviously significant as a lot of trainers spend a lot of their time playing online. While the bug will ruin the game for a lot of trainers if they're unlucky enough to come across it, it's important to note it won't corrupt your save data. So, even if it does affect you, you won't lose the hours you have invested into your journey around Galar.
Kaphotics shared some further details about the problem: "[The game will] also crash if received via trade, but only once -- just release it afterwards." This is in comparison to a Surprise Trade where the problem will continue.
Your save data won't be corrupted; the Pokémon you received from Surprise Trade is waiting to be received and will crash when the game tries to receive it. Can't launch any online features. It'll also crash if received via trade, but only once -- just release it afterwards.
— Kurt (@Kaphotics) February 12, 2020
Apparently the issue has already been escalated to the Pokemon Company, who presumably, due to the severity of the issue, are working to fix it right now. The issue was reportedly initially found by Twitter users architdate and Sibuna_Switch who also subsequently confirmed the bug's behaviour.